Do employers not have respect for basic work ethic?

A common job interview question is "Why do you want this job?" And the true answer, "Because it's a job," is not acceptable.

Based on what my parents and grandparents tell me about their employment history, it seems you used to be able to get a job on the basis that you need a job and the job in question is a job and you're capable of doing the job in question. Employers would hire their employees' kids in the summer solely on the basis that the kids needed summer jobs. They didn't need to convince the employer of why they wanted to shovel coke or pack meat, it was obvious that they needed to make money. Apparently they even used to have designated places where people who needed work and employers who needed day labourers would all go and match themselves up with each other. The employers would hire people simply because they were there and willing to work!

And yet, every job I've interviewed for, including fast food, has had a "Why do you want this job?" question.

When did this start happening? Why do they do it? It makes me wonder if today's employers don't have respect for basic work ethic, that they went and created a world where willingness to work to earn a living is not considered sufficient motivation.